Secondary battery including electrolyte additive

ABSTRACT

Disclosed is a secondary battery including an electrode assembly, which includes a cathode, an anode and a separator interposed therebetween, and an electrolyte, wherein the anode includes lithium titanium oxide (LTO) as an anode active material and the electrolyte contains a phosphate-based compound as an additive.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International ApplicationNo. PCT/KR2013/006132 filed Jul. 10, 2013, which claims priority fromKorean Application Nos. 10-2012-0075818 filed Jul. 12, 2012 and10-2012-0075153 filed Jul. 10, 2012, the disclosures of which are herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a secondary battery that includes anelectrode assembly including a cathode, an anode and a separatorinterposed therebetween and an electrolyte, wherein the anode includeslithium titanium oxide (LTO) as an anode active material and theelectrolyte contains a phosphate-based compound as an additive.

BACKGROUND ART

Technological development and increased demand for mobile devices haveled to rapid increase in the demand for secondary batteries as energysources. Among such secondary batteries, lithium secondary batterieshaving high energy density, high operating voltage, long cycle span andlow self-discharge rate are commercially available and widely used.

In addition, increased interest in environmental issues has recentlybrought about a great deal of research associated with electric vehicles(EV) and hybrid electric vehicles (HEV) as alternatives to vehiclesusing fossil fuels, such as gasoline vehicles and diesel vehicles, whichare a main cause of air pollution. Such electric vehicles generally usenickel-metal hydride (Ni-MH) secondary batteries as power sources.However, a great deal of study associated with use of lithium secondarybatteries having high energy density, high discharge voltage and stableoutput is currently underway and some are commercially available.

Lithium secondary batteries may be classified into lithium-ion batteriescontaining liquid electrolytes per se, lithium-ion polymer batteriescontaining liquid electrolytes in a gel form, and lithium polymerbatteries containing solid electrolytes, depending upon the type ofelectrolyte employed. Particularly, use of lithium-ion polymer or gelpolymer batteries is on the rise due to various advantages thereof suchas high safety owing to a low probability of fluid leakage, as comparedto liquid electrolyte batteries, and the possibility of achieving verythin and lightweight batteries.

A lithium-ion battery is manufactured by impregnating a liquidelectrolyte containing a lithium salt into an electrode assembly thatincludes a cathode and an anode, each being formed by applying an activematerial to a current collector, with a porous separator interposedbetween the cathode and anode.

Methods for fabricating a lithium-ion polymer battery are divided into afabrication method of a non-crosslinked polymer battery and afabrication method of a directly-crosslinked polymer battery, dependingupon the type of a matrix material for electrolyte impregnation.Acrylate- and methacrylate-based materials having high radicalpolymerization reactivity and ether-based materials having highelectrical conductivity are typically used as the polymer matrixmaterials. In particular, in directly-crosslinked polymer batteryfabrication, a battery is fabricated by placing a jelly-roll type orstack type electrode assembly composed of electrode plates and a porousseparator in a pouch, injecting a thermally polymerizable polyethyleneoxide (PEO)-based monomer or oligomer crosslinking agent and anelectrolyte composition into the pouch, and thermally curing theinjected materials. Manufacture of batteries in this manner isadvantageous in that electrode plates and separators of conventionallithium-ion batteries are used without change. However,directly-crosslinked polymer battery fabrication has problems in that acrosslinking agent is not completely cured and remains in theelectrolyte, increasing viscosity. This makes uniform impregnationdifficult, thereby greatly degrading battery properties.

A carbon-based material is typically used as an anode active materialfor lithium secondary batteries. However, the carbon-based material hasa low potential of 0V relative to lithium and thus reduces theelectrolyte, generating gases. Lithium titanium oxide (LTO) having arelatively high potential is also used as an anode active material forlithium secondary batteries to solve these problems.

However, when LTO is used as an anode active material, the LTO acts as acatalyst, generating a large amount of hydrogen gas during activationand charge/discharge processes, which causes a reduction in secondarybattery safety.

Thus, there is a great need to provide a technology that secures batterysafety by solving the above problems while maintaining overall batteryperformance.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the above andother technical problems that have yet to be resolved.

As a result of intensive studies and various experiments, the presentinventors discovered that desired effects are achieved when a secondarybattery including lithium titanium oxide (LTO) as an anode activematerial and a phosphate-based compound as an electrolyte additive isused. The present invention has been completed based on this discovery.

Technical Solution

In accordance with the present invention, the above and other objectscan be accomplished by the provision of a secondary battery including anelectrode assembly including a cathode, an anode and a separatorinterposed therebetween and an electrolyte, wherein the anode includeslithium titanium oxide (LTO) as an anode active material, and theelectrolyte contains a phosphate-based compound as an additive.

In a specific embodiment, the electrolyte may be, but is not limited to,any of a liquid electrolyte, a gel electrolyte and a solid electrolyte.Specifically, the electrolyte may be a liquid electrolyte or a gelpolymer electrolyte.

When the electrolyte is a liquid electrolyte, decomposition of theelectrolyte may be promoted by side reaction of the electrolyte with theanode active material, thereby generating gas as described above. Suchgas may cause safety problems of the secondary battery such as swellingor explosion. Thus, the secondary battery according to the presentinvention uses a liquid electrolyte with a phosphate-based compoundadded thereto to solve such problems.

When the electrolyte is a gel polymer electrolyte, the phosphate-basedcompound additive, which reacts as a crosslinking agent, is added to thegel polymer electrolyte. This provides effects of superior cyclecharacteristics while achieving electrode interface stabilization, thusgreatly inhibiting swelling caused by gas generation during storage athigh temperature. As a result, the effects of greatly improved batterylifespan and safety are also achieved.

Here, it is believed that, since the phosphate-based compound has highreactivity with radicals, it increases the extent of polymerizationreaction, thereby improving electrochemical stability of the finalelectrolyte. In addition, LTO used as an anode active material acts as acatalyst to promote crosslinking polymerization of the phosphate-basedcompound, thereby maximizing the effects described above.

Particularly, when the electrolyte is a gel polymer electrolyte, sidereactions of the electrolyte with the electrodes are reduced duringrepeated charge/discharge since an area of the electrolyte in contactwith the electrodes is reduced and swelling is also inhibited due to areduction in vapor pressure since the electrolyte is in a gel polymerform.

In one embodiment, the phosphate-based compound may include at least oneselected from the group consisting of a phosphate-based acrylate ofFormula (1), a pyrophosphate-based acrylate of Formula (2) and aphosphate-based urethane acrylate:

where R₁ and R₂ are each independently hydrogen, methyl or F and n is aninteger of 1 to 20.

The electrolyte may further contain a multifunctional compoundpolymerizable with the phosphate-based compound.

When a multifunctional compound polymerizable with the phosphate-basedcompound is additionally used as an electrolyte additive, themultifunctional compound and the phosphate-based compound can complementelectrochemical and mechanical characteristics of each other, therebyfurther improving overall characteristics of the battery.

Particularly, when a gel polymer electrolyte is prepared using both thephosphate-based compound and a multifunctional compound polymerizablewith the phosphate-based compound, physical properties with higherelasticity are achieved. That is, a phosphate-based compound, which hasa structure enabling easy coordination with lithium ions, thusexhibiting higher bonding force, and a multifunctional compound havinghigh elasticity are polymerized through crosslinking together, such thatthe phosphate-based compound and the multifunctional compound complementelectrochemical and mechanical characteristics of each other.

In an embodiment, the multifunctional compound may include at least oneselected from the group consisting of a (meth)acrylic acid estercompound, an unsaturated carbonic acid compound and a vinyl compound.

The (meth)acrylic acid ester compound may include a (meth)acrylatecompound having at least two acrylate groups per molecule and the(meth)acrylate compound may include a monomer of Formula (3) or anoligomer thereof:

where R³, R⁴ and R⁵ are each independently hydrogen or substituted orunsubstituted C₁-C₄ alkyl, and m is an integer of 1 to 20.

In addition, the (meth)acrylic acid ester compound may include, but isnot limited to, at least one selected from the group consisting ofdiethylene glycol diacrylate (Di(EG)DA), diethylene glycoldimethacrylate (Di(EG)DM), ethylene glycol dimethacrylate (EGDM),dipropylene diacrylate (Di(PG)DA), dipropylene glycol dimethacrylate(Di(PG)DM), ethylene glycol divinyl ether (EGDVE), ethoxylated(6)trimethylolpropane triacrylate (ETMPTA), diethylene glycol divinyl ether(Di(EG)DVE), triethylene glycol dimethacrylate (Tri(EG)DM),dipentaerythritol pentaacrylate (DPentA), trimethylolpropane triacrylate(TMPTA), trimethylolpropane trimethacrylate (TMPTM), propoxylated(3)trimethylolpropane triacrylate (PO(3)TMPTA), propoxylated(6)trimethylolpropane triacrylate (PO(6)TMPTA), poly(ethyleneglycol)diacrylate (PA1) and poly(ethylene glycol)dimethacrylate.

The multifunctional compound, together with the phosphate-basedcompound, may form various types of copolymers, for example, randomcopolymers, block copolymers, and graft copolymers.

The electrolyte may contain 0.1 to 1%, more specifically 0.1 to 0.5%, byweight of the multifunctional compound polymerizable with thephosphate-based compound, based on the total weight of the electrolyte.

The electrolyte may contain 0.01 to 30%, more specifically 0.01 to 20%,by weight of the phosphate-based compound, based on the total weight ofthe electrolyte.

If the content of the phosphate-based compound is excessively low whenthe electrolyte is a liquid electrolyte, the effects of improved safetyare not fully achieved. On the contrary, if the content of thephosphate-based compound is excessively high, overall batterycharacteristics may be degraded since the content of lithium salt isrelatively lowered although safety is improved.

If the content of the phosphate-based compound is excessively low whenthe electrolyte is a gel polymer electrolyte, gel polymers are noteasily formed such that the phenomenon of swelling of the batteryoccurring when a liquid electrolyte is used may worsen and formation ofa substrate having a desired thickness may be difficult. On thecontrary, if the content of the phosphate-based compound is excessivelyhigh, the density of gel polymers is increased and lithium ionconduction rate (or conductivity) is accordingly reduced, causingprecipitation of lithium, with the result that battery performance isreduced. In addition, viscosity is increased, such that there may bedifficulty in uniform application of the electrolyte to a correspondingportion.

The same is true when the multifunctional compound is added to thephosphate-based compound. Thus, the electrolyte may contain thephosphate-based compound and the multifunctional compound in a totalamount of 0.01 to 30%, more specifically 0.1 to 5%, based on the totalweight of the electrolyte.

The liquid electrolyte may include an electrolyte (plasticizer) and alithium salt. When the electrolyte is a gel polymer electrolyte, theelectrolyte may further include a polymerization initiator.

The electrolyte also serves as a plasticizer. Examples of theelectrolyte include aprotic organic solvents such asN-methyl-2-pyrollidinone, propylene carbonate (PC), ethylene carbonate(EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC), gamma-butyrolactone,1,2-dimethoxy ethane, tetrahydrofuran, 2-methyl tetrahydrofuran,dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide,dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate,phosphoric acid triester, trimethoxy methane, dioxolane derivatives,sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylenecarbonate derivatives, tetrahydrofuran derivatives, ether, methylpropionate and ethyl propionate. These materials may be used singly oras a mixture of two or more thereof.

The lithium salt is a material that dissolves and dissociates intolithium ions in the non-aqueous electrolyte. Examples of the lithiumsalt include LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃,LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,chloroborane lithium, lower aliphatic carboxylic acid lithium, lithiumtetraphenylborate and imides. These materials may be used singly or as amixture of two or more thereof.

The electrolyte may contain 0.01 to 30%, more specifically 0.1 to 20%,by weight of the lithium salt based on the total weight of solidcomponents included in the electrolyte.

Examples of the polymerization initiator may include azo compounds suchas 2,2-azobis(2-cyanobutane), 2,2-azobis(methylbutyronitrile),2,2′-azoisobutyronitrile (AIBN) and azobisdimethyl-valeronitrile (AMVN),peroxy compounds such as benzoyl peroxide, acetyl peroxide, dilaurylperoxide, di-tert-butyl peroxide, cumyl peroxide and hydrogen peroxide,and hydroperoxides. Specifically, AIBN, 2,2′-azobis(2,4-dimethylvaleronitrile) (V65), di-(4-tert-butylcyclohexyl)-peroxydicarbonate(DBC) or the like may be used as the polymerization initiator.

The polymerization initiator may decompose at a temperature of 40 to 80°C. to form radicals and may then react with monomers through freeradical polymerization to form a gel polymer electrolyte. Generally,free radical polymerization is carried out by sequential reactionsincluding an initiation reaction involving formation of transientmolecules having high reactivity or active sites, a propagation reactioninvolving re-formation of active sites at the ends of chains by additionof monomers to active chain ends, a chain transfer reaction involvingtransfer of the active sites to other molecules and a terminationreaction involving destruction of active chain centers. Of course,polymerization may also be carried out without a polymerizationinitiator.

In addition, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrroles, 2-methoxy ethanol,aluminum trichloride or the like may be added to the electrolyte. Whereappropriate, the non-aqueous electrolyte may further include ahalogen-containing solvent such as carbon tetrachloride or ethylenetrifluoride in order to impart incombustibility. Further, thenon-aqueous electrolyte may additionally include carbon dioxide gas inorder to improve high-temperature storage characteristics.

The secondary battery according to the present invention may be alithium-ion battery. The lithium-ion battery may be fabricated bymounting an electrode assembly in a battery case, injecting a mixture ofa phosphate-based compound, an electrolyte and a lithium salt into thebattery case, followed by sealing, and performing a formation process toactivate the battery and an aging process to stabilize the activatedbattery.

However, when the electrolyte is a gel polymer electrolyte, anactivation process is performed after gel reaction. When thephosphate-based compound is used as an additive to the electrolyte, amethod in which film formation is induced through wetting andcharge/discharge may be employed while the gel reaction is omitted. In abasic method, the battery is charged up to a level, at which anelectrochemical decomposition reaction of monomers may occur, anddegassing is then performed.

The formation process is a process that activates the battery byrepeating charge/discharge cycles. The aging process is a process thatstabilizes the battery activated in the formation process by allowingthe battery to stand for a certain period of time.

Conditions under which the formation process and the aging process arecarried out are not particularly limited and are adjustable withinconventional ranges well known in the art.

In a specific embodiment, the mixture is injected into the battery case(primary injection) and the battery structure is allowed to stand for acertain period of time (for example, 10 hours) such that uniformimpregnation of the mixture into the battery case is achieved. Thebattery is then charged for activation. In the charge process foractivation, gases generated during formation of a protective film forthe anode are removed. Thereafter, the battery is again allowed to standfor a certain period of time (for example, 12 hours) and charged foractivation, thereby completing battery fabrication.

The secondary battery according to the present invention may be alithium-ion polymer battery. Specifically, the lithium-ion polymerbattery may be fabricated using a method including (a) mounting anelectrode assembly in a battery case, (b) injecting a mixture of aphosphate-based compound, a polymerization initiator, an electrolyte anda lithium salt into the battery case, followed by sealing, and (c)polymerizing the phosphate-based compound to form a gel polymerelectrolyte.

Specifically, step (c) may include (c1) subjecting the battery tothermal curing, photocuring via irradiation with electron beams or gammarays, or a stabilization reaction at 30 to 80° C. to polymerize thephosphate-based compound, and (c2) performing a formation process toactivate the battery and an aging process to stabilize the activatedbattery.

Specifically, the crosslinking reaction may be carried out under inertconditions. Since the reaction of radicals with atmospheric oxygenserving as a radical scavenger is fundamentally blocked under inertatmosphere, it is possible to enhance the extent of reaction to a levelat which substantially no unreacted monomers are present. This preventsdegradation in charge/discharge performance caused by a large amount ofunreacted monomers remaining inside the battery.

The inert atmosphere conditions are not particularly limited. Knowngases with low reactivity can be used. For example, at least oneselected from the group consisting of nitrogen, argon, helium and xenonmay be used as inert gases.

Phosphate-based compounds are combined via the crosslinkingpolymerization reaction to form crosslinked polymers having athree-dimensional network structure, and the polymers are then uniformlyimpregnated with the electrolyte.

The crosslinked polymer electrolyte is electrochemically stable andtherefore can be stably present in the battery without being damagedeven after repeated charge/discharge cycles. As a result, it is possibleto improve battery safety and achieve excellent mechanical propertiessuch as elongation and bending properties. Further, battery performancedeterioration can be minimized due to continuous migration and transferof lithium ions through the polar gel polymer electrolyte.

The formation and aging processes are performed in the same manner asdescribed above. During the formation process, lithium ions that areliberated from lithium metal oxide used as the cathode upon charging ofthe battery migrate and intercalate into the carbon electrode used asthe anode. Here, compounds such as Li₂CO₃, LiO and LiOH, which areproduced by the reaction of highly-reactive lithium with the carbonanode, form a solid electrolyte interface (SEI) film on the anodesurface. In this case, an unreacted crosslinking agent may undergoadditional reaction.

In a specific embodiment, the mixture is injected into the battery case(primary injection) and the battery structure is allowed to stand for acertain period of time (for example, 3 hours) such that uniformimpregnation of the mixture into the battery case is achieved. Thermalpolymerization is then carried out under the above-specified conditions.The battery is then charged for activation. In the charge process foractivation, gases generated during formation of a protective film forthe anode are removed and a certain amount of supplementary mixture issecondarily injected into the battery case. Thereafter, the battery isagain allowed to stand for a certain period of time (for example, 12hours) and charged for activation, thereby completing batteryfabrication.

The secondary battery is generally fabricated by incorporating anelectrolyte into an electrode assembly including a cathode and an anodewith a separator interposed therebetween.

The cathode is prepared, for example, by applying a mixture of a cathodeactive material, a conductive material and a binder to a cathode currentcollector, followed by drying, and pressing. A filler may be added tothe mixture as needed.

The cathode current collector is generally manufactured to a thicknessof 3 to 500 μm. Any cathode current collector may be used withoutparticular limitation so long as high conductivity is provided withoutcausing chemical changes in the battery. Examples of the cathode currentcollector include stainless steel, aluminum, nickel, titanium, sinteredcarbon, or aluminum or stainless steel surface-treated with carbon,nickel, titanium or silver. The cathode current collector may includefine irregularities on the surface thereof so as to enhance adhesion tothe cathode active material. In addition, the cathode current collectormay be used in various forms such as a film, a sheet, a foil, a net, aporous structure, a foam and a nonwoven fabric.

Examples of the cathode active material include, but are not limited to,layered compounds such as lithium cobalt oxide (LiCoO₂) and lithiumnickel oxide (LiNiO₂) alone or substituted by one or more transitionmetals; lithium manganese oxides such as Li_(1+x)Mn_(2−x)O₄ (where0≤x≤0.33), LiMnO₃, LiMn₂O₃ and LiMnO₂; lithium copper oxide (Li₂CuO₂);vanadium oxides such as LiV₃O₈, LiFe₃O₄, V₂O₅ and Cu₂V₂O₇; Ni-site typelithium nickel oxides represented by LiNi_(1−x)M_(x)O₂ (M=Co, Mn, Al,Cu, Fe, Mg, B or Ga and 0.01≤x≤0.3); lithium manganese composite oxidesrepresented by LiMn_(2−x)M_(x)O₂ (M=Co, Ni, Fe, Cr, Zn or Ta and0.01≤x≤0.1) or Li₂Mn₃MO₈ (M=Fe, Co, Ni, Cu or Zn); LiMn₂O₄ wherein Li ispartially substituted by alkaline earth metal ions; disulfide compounds;and Fe₂(MoO₄)₃.

The conductive material is commonly added in an amount of 0.01 to 50% byweight, based on the total weight of the mixture including the cathodeactive material. Any conductive material may be used without particularlimitation so long as suitable conductivity is provided without causingchemical changes in the battery. Examples of the conductive materialinclude graphite such as natural or artificial graphite, carbon blackssuch as acetylene black, Ketjen black, channel black, furnace black,lamp black and thermal black, conductive fibers such as carbon fibersand metallic fibers, metallic powders such as carbon fluoride, aluminumand nickel powders, conductive whiskers such as zinc oxide and potassiumtitanate whiskers, conductive metal oxides such as titanium oxide, andpolyphenylene derivatives.

The binder is a component assisting in binding of an active material toa conductive material and a current collector. The binder is commonlyadded in an amount of 1 to 50% by weight, based on the total weight ofthe compound including the cathode active material. Examples of thebinder include polyfluorovinylidene, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene terpolymers (EPDM), sulfonated EPDM, styrenebutadiene rubbers, fluoro-rubbers and various copolymers.

The filler is a component optionally used to inhibit cathode expansion.Any filler may be used without particular limitation so long as thefiller is a fibrous material that does not cause chemical changes in thebattery. Examples of the filler include olefin-based polymers such aspolyethylene and polypropylene and fibrous materials such as glassfibers and carbon fibers.

For example, the anode is prepared by applying an anode active materialto an anode current collector, followed by drying and pressing. Theanode may further include other components as needed as described above.

The anode current collector is generally manufactured to a thickness of3 to 500 μm. Any anode current collector may be used without particularlimitation so long as suitable conductivity is provided without causingchemical changes in the battery. Examples of the anode current collectorinclude copper, stainless steel, aluminum, nickel, titanium, sinteredcarbon, copper or stainless steel surface-treated with carbon, nickel,titanium or silver, or an aluminum-cadmium alloy. Similar to the cathodecurrent collector, the anode current collector may include fineirregularities on the surface thereof so as to enhance bonding force tothe anode active material. In addition, the anode current collector maybe provided in various forms such as a film, a sheet, a foil, a net, aporous structure, a foam and a nonwoven fabric.

Lithium titanium oxide may be used as the anode active material asdescribed above.

Specifically, the lithium titanium oxide may be Li₄Ti₅O₁₂, LiTi₂O₄ or amixture thereof. More specifically, the lithium titanium oxide may beLi₄Ti₅O₁₂.

Examples of the anode active material may include a mixture of carbonsuch as non-graphitized carbon or graphitized carbon, metal compositeoxide such as Li_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1) orSn_(x)Me_(1−x)Me′_(y)O_(z) (Me: Mn, Fe, Pb or Ge; Me′: Al, B, P, Si,Group I, II and III elements of the Periodic Table or halogens; 0<x≤1,1≤y≤3, and 1≤z≤8), a lithium metal, a lithium alloy, a silicon-basedalloy, a tin-based alloy, metal oxide such as SnO, SnO₂, PbO, PbO₂,Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄ or Bi₂O₅, aconductive polymer such as polyacetylene and a Li—Co—Ni based material.

The secondary battery according to the present invention may befabricated in various forms. For example, the electrode assembly may beconstructed in a jelly-roll structure, a stacked structure, astacked/folded structure or the like. The battery may be structured suchthat an electrode assembly is installed inside a battery case made of acylindrical can, a prismatic can or a laminate sheet including a metallayer and a resin layer. Such a structure of the battery is widely knownin the art and therefore a detailed description thereof is omittedherein.

The secondary battery may be a lithium secondary battery.

The secondary battery may be used not only as a power source forsmall-scale devices but also as a power source for middle or large-scaledevices as described below.

The present invention also provides a battery module including thesecondary battery as a unit cell and a battery pack including thebattery module.

The battery pack may also be used as a power source for middle orlarge-scale devices that require high-temperature safety, long cyclespan and high rate properties.

Specific examples of the middle or large-scale devices include, but arenot limited to, power tools that are powered by electrical motors,electric vehicles (EVs) including hybrid electric vehicles (HEVs) andplug-in hybrid electric vehicles (PHEVs), electric two-wheeled vehiclesincluding electric bikes (E-bikes) and electric scooters (E-scooters),electric golf carts and power storage systems.

Advantages

Since lithium titanium oxide (LTO) is used as an anode active materialand a phosphate-based compound is used as an additive, a secondarybattery according to the present invention achieves electrode interfacestabilization, thereby preventing generation of gases and by-products.Thus, the secondary battery exhibits not only high safety but alsoimproved lifespan and high-power characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing comparison of cycle characteristics in a 45°C. chamber according to Experimental Example 2; and

FIG. 2 is a graph showing comparison of the extent of gas generationassociated with high-temperature storage lifespan in a 60° C. chamberaccording to Experimental Example 3.

BEST MODE

The present invention will now be further described through examples.However, it should be noted that the following examples are given onlyto exemplify the present invention without limiting the scope of theinvention.

Example 1

An anode active material (Li_(1.33)Ti_(1.67)O₄), a conductive material(Denka black) and a binder (PVdF) were added in a weight ratio of95:2.5:2.5 to NMP, followed by mixing, to prepare an anode mix. Theanode mix was then applied to a copper foil having a thickness of 20 μmto form a coating layer having a thickness of 60 μm, followed by rollingand drying, to produce an anode.

In addition, LiNi_(0.5)Mn_(1.5)O₄ as a cathode active material, aconductive material (Denka black) and a binder (PVdF) were added in aweight ratio of 95:2.5:2.5 to NMP, followed by mixing, to prepare acathode mix. The cathode mix was then applied to a copper foil having athickness of 20 μm, followed by rolling and drying, to produce acathode.

A polyethylene membrane (Celgard, thickness: 20 μm) was then interposedas a separator between the anode and the cathode to form an electrodeassembly. A liquid electrolyte with 1M LiPF₆ dissolved in an EC/EMCsolvent at ½ volume ratio, to which a phosphate-based acrylate (R₁ is Hand n is 1 in Formula (1)) was added as a phosphate-based material in anamount of 5% by weight based on the total weight of the electrolyte, wasinjected into a pouch, in which the electrode assembly was mounted, tofabricate a pouch battery.

Example 2

A pouch battery was fabricated in the same manner as in Example 1 exceptthat a pyrophosphate-based acrylate (R₁ is H and n is 1 in Formula (2))was used as a phosphate-based material.

Example 3

A pouch battery was fabricated in the same manner as in Example 1 exceptthat dipentaerythritol pentaacrylate (DPentA) was additionally added asa multifunctional compound to the electrolyte in an amount of 0.2% byweight based on the weight of the solvent.

Example 4

A pouch battery was fabricated in the same manner as in Example 2 exceptthat dipentaerythritol pentaacrylate (DPentA) was additionally added asa multifunctional compound to the electrolyte in an amount of 0.2% byweight based on the weight of the solvent.

Example 5

A pouch battery was fabricated in the same manner as in Example 1 exceptthat 2,2′-azoisobutyronitrile (AIBN) was added as a polymerizationinitiator to the electrolyte in an amount of 0.1% by weight based on theweight of the solvent after injection of the electrolyte andhigh-temperature reaction was then carried out at a temperature of 70°C. for 5 hours to prepare a gel polymer electrolyte.

Example 6

A pouch battery was fabricated in the same manner as in Example 2 exceptthat 2,2′-azoisobutyronitrile (AIBN) was added as a polymerizationinitiator to the electrolyte in an amount of 0.1% by weight based on theweight of the solvent after injection of the electrolyte andhigh-temperature reaction was then carried out at a temperature of 70°C. for 5 hours to prepare a gel polymer electrolyte.

Comparative Example 1

A pouch battery was fabricated in the same manner as in Example 1 exceptthat an electrolyte with no phosphate-based acrylate (R₁ is H and n is 1in Formula (1)) added thereto was used.

Comparative Example 2

A pouch battery was fabricated in the same manner as in Example 6 exceptthat an electrolyte was injected after a phosphate-based acrylate (R₁ isH and n is 1 in Formula (1)) was added to the electrolyte in an amountof 40% by weight.

Experimental Example 1

Batteries (with a design capacity of 265 mAh) fabricated in Examples 1to 6 and Comparative Examples 1 and 2 were subjected to a formationprocess at 2.75 V. The batteries were charged/discharged at a certainC-rate in a range between 1.6 V and 2.75 V to verify discharge capacity.Results are shown in Table 1 below.

TABLE 1 Discharge Capacity Ex. 1 259 mAh Ex. 2 255 mAh Ex. 3 256 mAh Ex.4 252 mAh Ex. 5 255 mAh Ex. 6 256 mAh Comp. Ex. 1 250 mAh Comp. Ex. 2230 mAh

Experimental Example 2

Cycle characteristics of batteries fabricated in Examples 1 and 3 andComparative Examples 1 and 2 were measured while the batteries werecharged/discharged at a C-rate of 5 C in a range between 1.6 V and 2.75V in a 45° C. chamber. Results are shown in FIG. 1.

Experimental Example 3

Batteries (with a design capacity of 265 mAh) fabricated in Examples 1and 3 and Comparative Examples 1 and 2 were subjected to a formationprocess at 2.75 V. The extent of gas generation by side reaction wasmeasured after the batteries were stored in an SOC of 100% at a hightemperature of 60° C. Results are shown in FIG. 2.

As can be seen from FIGS. 2 and 3, Comparative Example 1 generated anexcessive amount of gases and Comparative Example 2 significantlydegraded cycle characteristics, whereas Examples 1 to 6 according to thepresent invention generated a small amount of gases, securing highsafety, and also exhibited superior cycle characteristics.

As is apparent from the above description, a secondary battery accordingto the present invention has a variety of advantages. For example, sincelithium titanium oxide (LTO) is used as an anode active material and aphosphate-based compound is used as an additive, the secondary batteryachieves electrode interface stabilization, thereby preventinggeneration of gases and by-products. Thus, the secondary batteryexhibits not only high safety but also improved lifespan and high-powercharacteristics.

It will be apparent to those skilled in the art that variousmodifications and variations are possible in light of the above teachingwithout departing from the scope of the invention.

The invention claimed is:
 1. A secondary battery comprising an electrode assembly comprising a cathode, an anode and a separator interposed therebetween, and an electrolyte, wherein the anode comprises lithium titanium oxide (LTO) as an anode active material, and the electrolyte is a liquid electrolyte, which contains a phosphate-based compound as an additive, wherein the phosphate-based compound comprises at least one selected from the group consisting of a phosphate-based acrylate of Formula (1), a pyrophosphate-based acrylate of Formula (2) and a phosphate-based urethane acrylate:

where R₁ and R₂ are each independently hydrogen, methyl or F, and n is an integer of 1 to
 20. 2. The secondary battery according to claim 1, wherein the electrolyte contains 0.01 to 30% by weight of the phosphate-based compound based on the total weight of the electrolyte.
 3. The secondary battery according to claim 1, wherein the liquid electrolyte comprises an electrolyte serving as a plasticizer and a lithium salt.
 4. The secondary battery according to claim 3, wherein the electrolyte contains 0.01 to 30% by weight of the lithium salt based on the total weight of solid components included in the electrolyte.
 5. A battery module comprising the secondary battery according to claim 1 as a unit cell.
 6. A battery pack comprising the battery module according to claim
 5. 7. A device comprising the battery pack according to claim
 6. 8. The device according to claim 7, wherein the device is an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or a power storage system. 